1. Field of the Invention
This invention relates to a telephone apparatus, a modem device, a computer apparatus and a communication terminal device, and more particularly, is suitably applied to a telephone apparatus, modem device, computer apparatus and communication terminal device which are used as communication apparatuses which are connected to two or more different communication networks by utilizing a public telephone circuit.
2. Description of the Related Art
A public telephone circuit connects between communication parties by using telephone apparatuses as terminal devices for communicating between the parties by means of sound. However, in recent years, with the introduction of the Internet, computers are connected through the public telephone circuit so that not only sound information but also character and image information can be transmitted. If the conventional communication network in which the telephone apparatuses are connected is considered as a first network, the communication network in which the computer apparatuses are connected can be referred to as a second communication network.
The second communication network requires a structure for connecting between computer apparatuses. In other words, data is transmitted between the computer apparatuses by means of a digital signal. However, since an information signal is transmitted in the state of an analog signal on the public telephone circuit, a device which is referred to as a modem device is required between the computer apparatuses and the telephone circuit.
The information signal comprising the digital signal sent from the computer apparatus is inputted to the modem device. The modem device digital/analog-converts the information signal into an analog signal and transmits the obtained analog signal to the computer apparatus on the other party side through the public telephone circuit. The computer apparatus on the other party side receives an information signal comprising an analog signal, which is sent through the public telephone circuit, and inputs the information signal to the modem device. The modem device analog/digital-converts the inputted information signal to restore the signal to a digital signal, and supplies the obtained digital signal to the computer apparatus.
Each computer apparatus is capable of exchanging information signals through the public telephone circuit which can send only an analog signal, by attaching the modem device between the computer apparatus and the telephone circuit to analog/digital-convert or to digital/analog-convert the information signal. Since the computer network is formed by using an established public telephone circuit, various information can be transmitted to remote places such as other countries at relatively cheap cost.
By the way, there is a problem in that devices which are required to connect two different kinds of communication networks differ from each other; one device can not function the same way as another device.
For example, in communication by means of sound, an apparatus which is required for connecting with the public telephone circuit is a telephone apparatus. On the other hand, in the case of transmitting not only sound information but also character and image information for communication, a modem device and a computer apparatus are required as devices for connecting with the public telephone circuit; no telephone apparatus is required. Consequently, in the case of communication by means of character and image information in addition to communication by means of only sound information, the telephone apparatus, the modem device and the computer apparatus must be provided individually with the result that the overall structure and cost tend to increase.